As is known, in a hydrofoil seacraft, the hull of the craft is liftd out of the water by means of foils which are carried on struts and usually pass through the water beneath the surface thereof. In passing through the water, and assuming that sufficient speed is attained, the foils create enough lift to raise the hull above the surface and, hence, eliminate the normal resistance encountered by a ship hull in passing through the water.
In the usual case, there are forward and aft foils, both provided with control flaps similar to those used on aircraft. The other essential element is the rudder which pierces or is submerged beneath the surface of the water and is either forward or aft of the craft, depending upon its design. In most hydrofoils, the flaps are used primarily to cause the craft to ascend or descend and to control the craft about its pitch and roll axes; however they can also be used in combination with the rudder to bank the craft about its roll axis during a turn. The flaps are also used to stabilize the craft during movement on water. For example, pitching or rolling motions can be minimized by proper counterbalancing movement of the flaps. A typical control system for hydrofoils is shown, for example, in copending application Ser. No. 302,559, filed Oct. 31, 1972 and assigned to the Assignee of the present application.
In the system described in the aforesaid copending application Ser. No. 302,559, provision is made for sensing the height of the bow of the craft above the surface of the water during foil-borne operation and for producing an electrical signal proportional thereto. This signal is compared with a signal proportional to desired height as determined by the pilot; and if the two signals are not the same, the control flap on the forward foil is adjusted until actual height is equal to desired height. Additionally, a vertical gyro produces a signal proportional to the pitch angle of the craft for controlling both the forward and aft flaps. The arrangement is such that if, for example, the bow of the craft should dip, the forward flap is rotated downwardly and the aft flaps are rotated upwardly to counteract the pitching motion.
Theoretically, it is possible to cause the hydrofoil to take off with a system of the type described above by having the pilot simply set the desired height setting for foil-borne operation while the craft is hull-borne and advancing the craft's throttle. Under these circumstances, comparison of the desired and actual height signals would cause a large error signal which would rotate the forward flap downwardly, causing an excessive drag condition and requiring an extremely high takeoff thrust. At the same time, if a system of this type were employed, the bow of the craft would leave the water in a transition from hull-borne to foil-borne operation at a relatively large pitch angle, causing the aft flaps to deflect downwardly. This, again, would cause excessive drag during takeoff conditions. As a result, the transition from hull-borne to foil-borne operation in many prior art hydrofoils often required special control surface deflections that are used only for this purpose. These special control surface deflections, in turn, were effected by utilizing special levers or lever settings adjusted by the operator prior to making the transition from hull-borne to foil-borne operation. Needless to say, this forces undesirable operations and procedures upon the pilot which are not directly involved in either foil-borne or hull-borne operation.